Two common forms of distortion in digital data transmission systems are phase jitter and frequency offset. Phase jitter is a time-varying shift in the instantaneous phase of a modulated signal and is usually repetitive in some systematic manner. Frequency offset occurs when the modulating and demodulating carriers differ in frequency.
A number of techniques have been developed for combatting phase jitter and frequency offset. For example, where the demodulating carrier is derived from a pilot frequency transmitted over the transmission line, no frequency offset occurs, although the received waveform may be quite different from the transmitted waveform since the phase of the carrier is not adjusted. The reader is referred to U.S. Pat. No. 3,813,598 (Stuart) which discloses a carrier recovery system which utilizes a single pilot tone. Reference is also made to U.S. Pat. No. 3,849,730 (Ho) which likewise deals with the problems of phase jitter and frequency offset in a vestigial sideband amplitude modulation system. The Ho system employs a quadrature pilot tone added to the transmitted data signal. At the receiver, the carrier frequency, frequency offsets and phase jitter are estimated from the pilot tone which is separated from the data signal using a quadrature demodulator and associated circuitry. The requirement of a pilot tone has obvious disadvantages.